Uncured elastomers such as natural rubber, butyl rubber, EPDMs, polybutadiene, etc., may be crosslinked or vulcanized by the use of sulfonic accelerators which react with the carbon of the unsaturated bonds in the polymer molecules to form in effect a thermoset product which can no longer be fabricated or worked except by machining or similar techniques. These vulcanized polymers have found wide utility because of the significant improvement in physical properties by cross-linking. Thus, by vulcanizing rubber, elasticity, impact resistance, flexibility, and many other properties are either introduced or improved.
Recently, a class of polymers has been developed which, although they are crosslinked, have a softening point or softening range of temperatures and may even be dissolved in various solvents. At normal use temperatures these polymers behave similarly to crosslinked polymers. At elevated temperatures, however, they are readily deformed and worked in the same manner as thermoplastic resins. Such polymers are said to be physically crosslinked. An example of such materials are ionic hydrocarbon polymers (ionomers). These products owe their unique properties to the fact that cross-linking is accomplished by ionic rather than covalent bonding between molecules of the polymer. Typical of these ionomers are sulfonated polymers wherein the sulfonate is present in side chains in the form of sulfonic acid groups, sulfonic esters, or the corresponding metal salts of the sulfonic acid groups.
Methods of sulfonating polymers are well-known in the art. For example, aromatic containing polymers are sulfonated by the method described in U.S. Pat. No. 3,072,618, wherein a complex of a lower alkyl phosphate and SO.sub.3 is used as the sulfonating agent. These sulfonated aromatic polymers have generally been sulfonated to sufficient extent to be water-soluble in the form of their alkali metal salts. Other sulfonated polymers have been prepared by copolymerizing a styrene sulfonic acid salt with other monomers to form plastic polymers containing ionic crosslinks (see, e.g., U.S. Pat. No. 3,322,734). Natural rubber has been sulfonated by complexing chlorosulfonic acid with ethers or esters and reacting the complex with the rubber in solution (see, e.g., German Patents Nos. 582,565; 550,243 and 572,980). Polyolefins such as polypropylene, polyethylene, etc., have similarly been sulfonated utilizing complexes of lower alkyl phosphorus compounds and SO.sub.3 ; see, e.g., U.S. Pat. No. 3,205,285, which teaches that the dyeability of polypropylene and similar polymers may be improved by reacting the polymer fiber with the SO.sub.3 complex. The reaction of such treated fibers with alkali salts improves their dyeability. Further, it has recently been discovered that both plastic and elastomeric aromatic containing polymers (e.g., styrene-butadiene rubber) and non-aromatic polymers, in particular, olefinically unsaturated polymers such as butyl rubber and ethylene-propylene-diene terpolymers (commonly known as EPDMs) may be sulfonated to make physically cross-linked materials (see U.S. Application Ser. Nos. 806,052 and 877,849, incorporated herein by reference).
In general, the polymers mentioned above which are capable of sulfonation and are therefore useful in the instant invention are produced by solution polymerization, i.e., the monomeric reactants are dissolved in a suitable solvent and subjected to polymerization in the presence of either a Friedel-Crafts or a Ziegler-type catalyst. Ziegler-type catalysts are well-known in the art and typically include a compound which is preferably a halide of a transition metal, (e.g., titanium tetrachloride or vanadium tetrachloride) together with, as cocatalyst, an organometal compound (e.g., an organoaluminum compound such as diethyl aluminum chloride). Generally, the molar ratio of the cocatalyst to the catalyst is in the range of 1:1 to 16:1, preferably 1.5:1 to 7:1. The total amount of catalyst composition employed in the polymerization reaction may vary depending upon the particular components of the catalyst system but is generally in the range of about 0.01 to about 0.1 parts, preferably about 0.05 parts per hundred parts by weight of solvent.
Friedel-Crafts catalysts are also well-known in the art. For a good discussion of these types of catalysts, see the Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Edition, Interscience Publishers 1966, Vol. 10, pp. 158-163. In general, this type of catalyst may be classified as a Lewis acid and may be divided into six basic groups on the basis of chemical constitution: (1) acidic halides; (2) metal alkyls and alkoxides; (3) proton acids; (4) acidic oxides and sulfides, modified zeolites; (5) cation-exchange resins, and (6) metathetic cationforming agents. The most commonly used of these types are the acidic halides. Typical examples of acidic halides include AlCl.sub.3, AlBr.sub.3, BeCl.sub.2, CdCl.sub.2, ZnCl.sub.2, BF.sub.3, BCl.sub.3, BBr.sub.3, TiCl.sub.4, FeCl.sub.3, etc. When these types of catalysts are employed, the amounts used may, of course, vary depending upon the polymer to be produced but are generally in the range of about 0.01 to about 10.0 parts, preferably about 0.05 to about 0.5 parts, typically 0.2 parts per hundred parts by weight of solvent.
The solvent employed in the solution polymerization must be a nonreactive reaction medium. Typically, a saturated aliphatic hydrocarbon such as heptane, pentane, and hexane, or a chlorohydrocarbon such as tetrachloroethylene, methylchloride, ethylchloride, etc is employed. For purposes of this invention, an aromatic hydrocarbon such as toluene, is preferably not employed since aromatic rings may be subjected to sulfonation reactions at higher temperatures. All steps in the polymerization reaction are preferably carried out in the absence of extraneous amounts of oxygen, moisture, carbon dioxide or other harmful materials. Preferably, all reactants and catalysts should be pure and dry and blanketed with inert gas such as nitrogen or methane.
Reaction temperatures and pressures will, of course, vary to some extent depending on the polymer being prepared, and in any event these reaction conditions are not critical to functioning of the instant invention. At the conclusion of the polymerization reaction, the reactor effluent, as withdrawn, will normally include the following components: (a) the product polymer which is typically dissolved in the inert organic solvent to form a solution containing about 1 to about 50 parts, typically 3 to 10 parts of polymer per hundred parts of solvent; (b) unreacted monomer(s); and (c) some active and some spent catalyst.
In practice, the reactor effluent is normally treated by catalyst deactivation by addition of water, an alcohol or similar material; deashing; steam stripping; stabilizing; and drying.
Numerous problems may be encountered in attempting to sulfonate polymers prepared by solution polymerization. If the recovered polymer is to be stored prior to sulfonation, one difficulty arises in redissolving the baled stock prior to post-polymerization modification. Another difficulty resides in the fact that the stabilizers and processing aids which must be added in the normal finishing operation tend to react with the sulfonating agents resulting in greatly decreased efficiency of reaction and in unwanted side products. If it is decided to sulfonate freshly polymerized cement, difficulties arise from the fact that the polymer cement after polymerization contains substantial quantities of unreacted monomers and active catalyst. Unless the catalyst is deactivated, unwanted additional polymerization is likely to occur. However, if water or alcohols, agents used commercially as deactivating agents, are employed, the polymer cement is difficult or impossible to sulfonate since the catalyst deactivating agents will also deactivate the sulfonating reagent.